Synergistic effects of nitrogen doping in waste-fruit peel–derived activated carbons for supercapacitors and water treatment

Abstract

Developing sustainable carbon materials from agricultural waste offers promising solutions to both energy-storage and water-treatment challenges. This study reports the synthesis of a nitrogen-doped Tondolo-derived activated carbon (N-TAC), an underexplored African wetland sedge whose fibrous morphology directs pore architecture. Rather than conventional dry mixing, K₂CO₃ and urea were co-dissolved and intimately blended with the finely reduced Tondolo precursor. This reagent-level homogeneity enabled a single pyrolysis at 700 °C in which carbonisation, chemical activation, and nitrogen doping proceeded concurrently. The N-TAC Near-surface nitrogen functionalities (pyridinic, pyrrolic, and graphitic) proved decisive in enhancing both electrochemical and adsorptive performance. Three-electrode characterization in 1 M NaNO₃ revealed coexisting EDLC and pseudocapacitive mechanisms, with a low ESR of 2.05 Ω and relaxation time constant of 0.87 s confirming rapid ion diffusion kinetics. Trasatti deconvolution and Dunn's method reveal that pyridinic and pyrrolic sites selectively amplify pseudocapacitive contributions (63–71%) in the positive potential window, a window-dependent asymmetry not previously reported for neutral-electrolyte biomass carbons. The assembled symmetric device delivered 12.1 Wh kg⁻¹ with 94% capacitance retention after 10 000 cycles. For water remediation, N-TAC achieved 99.9% methylene blue removal at 50 ppm with an adsorption capacity of 557.6 mg g⁻¹, governed by chemisorption via electrostatic and π–π interactions, best described by pseudo-second-order kinetics (R² = 0.97) and the Temkin isotherm (R² = 0.958). These results position N-TAC as a competitive sustainable material at the water–energy nexus, with future perspectives including binder-free electrode fabrication, electrochemical degradation coupling, and regenerable architectures for long-term sustainability.